A desirable group of polymers are those that retain excellent mechanical properties at high temperatures. However, polymers in this group often melt at very high temperatures or decompose without melting. In addition their viscosities in the melt phase are extremely high. Therefore, these polymers are considered to be intractable, that is, not melt processable. Thus, forming these polymers into shaped articles is expensive at best and impossible in many cases.
For example, nylons of hexamethylene diamine and terephthalic acid exhibit excellent temperature resistance but cannot be melt-spun or molded because they decompose before their crystalline melting temperatures are reached. Likewise, many other wholly aromatic polymers such as polyimides of pyromellitic anyhydride and aromatic diamines cannot be melt-processed in polyamic acid or fully imidized form. Powder processing and sintering techniques have been used to process such intractible polymers into useable articles.
Specifically, polymeric particulate matter can be compacted into green bodies and sintered thereafter. Compaction behavior of polymeric particulates is referred to in "Compaction of Polymer Powders" by G. W. Halldin et al. SPE, 39th ANTEC, Boston, MA, May 4-7, 1981, pages 353-355 herein incorporated in its entirety. Halldin studied the density of green bodies as a function of compaction pressure, and among other things, concluded that the strength of an unsintered body is determined by its density. Generally speaking, there is an upper limitation of compaction pressure where increasing the pressure will not increase the density of the object, i.e., plateau density.
Halldin concluded that the compressibility of polymer powders can be best modeled by the equation: EQU .rho.*=.rho.*.sub.p -(.rho.*.sub.p -.rho.*.sub.a) exp (-kP)
wherein .rho.*=.rho..sub.g /.rho..sub.t, .rho..sub. =green density, gm/cm.sup.3, and .rho..sub.t =theoretical density, gm/cm.sup.3 ; .rho.*.sub.p =relative plateau density; .rho.*.sub.a =relative apparent density; k=compaction constant; and P=compaction pressure. The compaction constant, k, has units which are the reciprocal of the pressure units, for example, if P is expressed as Pa, k is expressed as Pa.sup.-1 or if P is expressed as psi, k is expressed as psi.sup.-1. According to the equation, as the compaction constant of a green body, k, approaches a value of 1.0 MPa.sup.-1, less compaction pressure, P, is required to produce a green body having a green density, .rho..sub.g, which approaches its theoretical (or true) density, .rho..sub.t according to "Powder Surface Area And Porosity" by S. Lowell & J. E. Shields, Chapman & Hall, 3rd. ed., 1991, p. 227. However, as the value of k decreases, more compaction pressure is required to produce a green body having a density which approaches the theoretical density. Some polymer powder, e.g., ultra high molecular weight polyethylene (UHMWPE), are readily compressed and exhibit k values approaching 1 MPa. Other polymer powders are not as readily compressible and exhibit k values as low as 0.01 MPa. The apparent density as measured by ASTM No. D-1895-89 is the density that the powder exhibits in its free flowing state.